Effects Of Understory Vegetation On The Pool And Dynamics Of Soil Organic Carbon In Poplar Plantations

Long-term and multi-rotation management with single species/clone has the risk of causing the decline of soil fertility,which seriously affected the sustainable management in poplar plantations.As a solution,increase in soil organic matter might play key roles in improving soil fertility.In the poplar plantation ecosystem,soil organic matter is mainly derived from litter return and decomposition of poplar and diversified understory vegetation.As traditional managing pattern,the understory vegetations were generally removed to reduce nutrient and water competition with poplar trees in the plantations.While recent researches revealed that understory vegetation might play crucial roles on soil nutrient cycling and help ecosystem stability.Therefore,scientific management of understory vegetation has become increasingly important for the sustainable management of poplar plantations.However,it is still unclear what roles the species and diversity of understory vegetation play in litter decomposition,soil organic formation,and system stability.Detailed researches were necessary to facilitate the understanding of the roles and functions of the species and diversity of understory vegetation.In this paper,a comparing study with the adjusting and controlling of the species and diversity of understory vegetation was conducted to study the influences and functions in young poplar plantations.Forest soils were collected in different growing seasons to analyze the effects of understory vegetation treatments on soil organic carbon pool and active organic carbon components.In addition,an incubation experiment with corresponding to the field experiments was also set up to investigate the effects of species and diversity of understory vegetation on the decomposition process of mixing litter of poplar with different understory vegetation.The field experiment results show that understory vegetation managements generally impact soil organic carbon pool in the soil surface layer(0~5cm)after one year.In defoliation season,the organic carbon content in the 0~5 soil layer of the undergrowth vegetation removal was reduced by 10.82% compared with the leaf-expanding season,while the natural vegetation retention was reduced by 3.71%.In defoliation season,the soil labile organic carbon content and the proportion of soil labile organic carbon in total organic carbon of the understory vegetation retention were 1.3 times and 1.2 times higher than the undergrowth vegetation removal.The water-soluble organic carbon content,microbial biological carbon content and easily oxidized organic carbon content of 0~5cm soil layer in the understory vegetation retention were 1.58 times,1.34 times and 1.05 times higher than the undergrowth vegetation removal.In different seasons,the activity of beta-glucosidase was lower in soil of undergrowth vegetation removal than soil of understory vegetation retention,while the activity of polyphenol oxidase was not significantly different.The content of labile organic carbon of organic carbon was 19.12% and 21.64% higher in Echinochloa crusgalli than in nitrogen-fixing planting.There was no difference in soil enzyme activity between the two forest structures in different growth seasons.It shows that the different types of understory vegetation only have significance influence on the content of labile organic carbon,but have no significance influence on other organic carbon components.It may be caused by the short management time of understory vegetation and the comprehensive influence of natural factors.The soil labile organic carbon of different vegetation diversity showed a significant difference in defoliation season.The content of water-soluble organic carbon,microbial biological carbon and easily oxidized organic carbon were 1.23 times,1.28 times and 1.55 times,respectively,higher in understory vegetation retention than in Echinochloa crusgalli during leaf-expangding season.In defoliation season,the soil water-soluble organic carbon content was 1.24 times higher in understory vegetation retention than Echinochloa crusgalli.In the fast-growing season,the activities of beta-glucosidase and polyphenol oxidase were higher in understory vegetation retention than Echinochloa crusgalli.Mixed decomposition of poplar leaves with litter of different undergrowth plants significantly affected the decomposition process.The residue rate,half decomposition time and 95% decomposition time of litter were P-N<P-M<P-E<P.The total litter carbon,soluble sugar,starch,cellulose and lignin weight loss rate and soil labile organic carbon and their components content of P-N treatment were higher than those of P treatment,and the total litter carbon of poplar P-M treatment was higher than that of poplar P-E treatment.The weight loss rate of soluble sugar,starch and lignin and the content of active organic carbon and its components in soil were higher.Compared with the mixed decomposition of P treatment with the addition of single plant and diversified plant,the total weight loss rate of litter residues,soluble sugar,starch,cellulose and lignin in P-N treatment and the content of soil labile organic carbon and its components were higher than those in P treatment.During the rising stage of enzyme activity during litter decomposition,the activities of beta-glucosidase and polyphenol oxidase in plants and soils were as follows: P-N>P,P-M>P-E and P-N>P-E.The order of cumulative soil respiration emissions during litter decomposition was P-E>P-M>P-N>P>CK.Therefore,after the removal of undergrowth vegetation,the content of labile organic carbon,microbial activity and activities of carbon conversion-related enzymes in the soil decreased.Among the forests with different types of undergrowth vegetation,the decomposition rate and nutrient release of litter treated by nitrogen-fixing plants are higher,which is conducive to the supplement of soil labile organic carbon and the maintenance of the stability of soil organic carbon pool.Compared with the main undergrowth vegetation treatment,the forest treated with the preservation of diversified undergrowth vegetation has higher litter decomposition rate and carbon release,lower cumulative soil respiration emissions,which is more conducive to the reduction and accumulation of soil organic carbon pool.